The present invention relates to an ignition timing control system having the functions of detecting a knocking by a vibration or a noise generated in or outside of a cylinder of an internal combustion engine by the pressure therein and retarding the ignition timing if a knocking is detected.
In recent years, what is called the knocking feedback system for detecting a knocking caused in an internal combustion engine and retarding the ignition timing has been studied variously. Such a system will be described briefly. Specifically, a vibration or a noise caused in or outside of an internal combustion engine is detected by the pressure in the cylinder, and if the vibration or the noise, as the case may be, exceeds a set level (a knocking decision level), a knocking is decided thereby to generate a knocking signal. When this knocking signal is generated, the ignition timing is retarded, while in the absence of a knocking signal, the ignition timing is advanced, so that the ignition timing is always controlled to a knocking limit or thereabouts thereby to improve the fuel efficiency and output performance of the engine.
In this knocking feedback system, the amount of retard per knocking upon detection of a knocking is predetermined and generally approximately 1.degree. CA (crank angle). This retard amount per knocking is a most important factor affecting the controllability of the ignition timing. This will be explained with reference to FIG. 1. As seen from FIG. 1, in order to improve both the ignition timing controllability under normal conditions and the ignition timing response under transient conditions, it is absolutely necessary to discriminate the normal and transient conditions and switch the retard amount per knocking therebeween.
Conventionally, however, for lack of means for discriminating the normal and transient conditions accurately, it has so far been impossible to switch the retard angle per knocking. In the method of switching the retard amount by detecting the acceleration of the engine, for instance, an unnecessarily large retard may be caused by a setting of a large retard, often deteriorating the engine acceleration performance, due to the fact that the difference of engines or environmental conditions may lead to a small knocking even under an accelerated state. For this reason, the retard amount is conventionally set at a compromise (such as 1.degree. CA) between the normal and transient conditions, with the result that the performance is deteriorated unavoidably under both normal and transient conditions.